Organically modified layered silicates have been widely studied for the
past decade as property enhancers for polymeric materials. Various studies
report improvement in mechanical (Hambir et al., 2001; Fornes et
al., 2001; Oya et al., 2000; Ke and Stroeve, 2005), thermal
(Beyer and Eupen, 2001; Gilman, 1999), flame resistance (Beyer and Eupen,
2001; Gilman, 1999) and barrier (Kojima et al., 1993b; Messermith
and Giannelis, 1995) properties of thermoplastics by addition of organically
modified layered silicates (Esfandiari et al., 2007; Saito et
al., 2006) to polymer matrices. These modified thermoplastic systems
are called Polymer-Layered Silicate Nanocomposites (PLSN). Due to this
property (Okamoto, 2006) enhancement at low filler content (2-6 wt. %),
PLSN systems have drawn tremendous attention. In general these PLSN systems
possess several advantages including; they are lighter in weight compared
to conventionally filled (Ton-That et al., 2006) polymers due to
property enhancement even at small clay loadings; they have enhanced flame
retardence and thermal stability and they exhibit outstanding barrier
properties without requiring a multi-layered design, allowing for recycling.
A few of those are explained as follow:
||Bayer AG, Germany developed nylon 6 nanocomposites for
transparent barrier film packaging. Bayer is marketing two grades
of nanocomposites, Durethan LPDU 601-1 and LPDU 601-2, which in comparison
to neat nylon offers decrease in the Oxygen Transmission Rate (OTR)
by 50%. The nanocomposites are made in the reactor using nano-clay
||Montell North America, General Motors R and D in Warren, Mich. and
Southern Clay have jointly developed Thermoplastic polyolefin (TPO)
based nanocomposites for use in the molding of body-side claddings
and step assist for the GMC Safari and Astro vans in 2002 (Fig.
||Unitika Co. of Japan developed Nylon 6 nanocomposite (Nylon M2350)
using synthetic clay as reinforcement during polymerization. Nylon
M2350 has been used by Mitsubishi Motors for an engine cover on its
GDI models, where the nanocomposite is said to offer a 20% weight
reduction and excellent surface finish.
||Japan`s Ube Industries developed a commercial nanocomposite of nylon
6 and nylon 6/66 copolymer (NCH) for film and structural applications.
Commercial applications of NCH include nylon 6 barrier films for food
packaging. NCH has also been used by Toyota Motor Company for manufacturing
the timing-belt cover on the Toyota Camry (Fig. 1b).
Most of the commercial applications of polymer-clay nanocomposites use
a polar polymer like (Nylon, EVA etc.) and the applications using non-polar
polymers like PE and PP are limited. The main reason for this is the poor
compatibility between a non-polar polymer and the organo-clay. Polyolefins
like PE and PP cover almost 70% of the thermoplastic market with major
applications in the automotive and packaging industry. These are the applications
where further improvement in stiffness, impact, thermal and barrier properties
without a significant increase in weight is always pleasing and wanted.
Reports on enhancement of barrier, mechanical and thermal properties on
addition of clay to a polymer have thus opened new fields of research,
involving improving compatibility between the polymer and the organo-clay,
in the polyolefin industry (Table 1).
Polyethylene (PE) counts for approximately 60% of the polyolefin market
and is thus one of the most researched thermoplastic and is also the focus
of the present research. A description of polyolefin, organo-clays and
the interaction between the two is described as:
Polyethylene: Polyethylene (PE) is one of the most widely used
commercial thermoplastics in the world today. PE counts for nearly 40%
of the total thermoplastic production in the United States (Equistar Chemical
Corporation Technical Bulletin). The low cost (~49 cents Lb-1)
of PE, plus versatility of attainable properties with modifications in
its molecular weight and chain architecture has generated a plethora of
commercial applications for PE.
PE can be classified into three main categories according to the architecture
of its main chain. The three principal types of PE, which are used extensively
in commercial film processing, are, high-density polyethylene (HDPE),
low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE).
Some typical applications for PE are, LDPE- bags, textile products, moisture
barriers, greenhouses, cable insulation; HDPE- bottles, pails, tubes,
caps, uses where injection molding of complex shapes is required but low
load is required, film, sheet, wire and cable insulation, pipes and drums
(Equistar Chemical Corporation Technical Bulletin). A large portion of
the polyolefin produced is consumed as film. Due to its low cost, high-density
polyethylene finds acceptance as a wrapping material for food products.
HDPE is known to have poor barrier properties for gases, organic solvents
and hydrocarbons (Gonzales-Nunez et al., 2001; Yen et al.,
1997). Reports on enhancement of barrier, mechanical and thermal properties
on addition of clay to a polymer have opened new fields of research in
the polyolefin industry. Polyolefin`s, being non-polar, show poor compatibility
with modified clays. Various authors Hambir et al. (2001), Wang
et al. (2001), Kodgire et al. (2001) and Hasegawa et
al. (1998) reported on the dispersion of clay platelets in polyolefin`s
by addition of a compatibilizer such as maleated polypropylene or maleated
|| Step assists on the GMC Safari developed by GM. a and
b) Timing belt cover
Clay: PLSN systems are usually made of two components; the base
resin and a modified layered silicate (clay). A potential third component
is a compatibilizer. Modified layered silicates are composed of silicate
layers that can intercalate organic polymer chains if appropriate ionic
or hydrogen bonding groups are present on the polymer. For example, montmorillonite
is a 2:1 type layered silicate and is the most commonly used filler in
PLSN systems (Brindley and Brown, 1980). 2:1 layered silicates are composed
of an octahedral alumina or magnesia sheet sandwiched between two tetrahedral
sheets of silica. The silica sheets have Na+, Ca2+,
or K+ ions on their surfaces. The combined thickness of the
two silica and one alumina or magnesia sheet is about 0.95 nm (Brindley
and Brown, 1980).
In its pristine form the clay is present as a crystal which is made up
of stack of silica sheets (platelets) along the (002) plane of the crystal.
The presence of positive ions on the surface of the silica sheets increases
the d-spacing in the (002) direction of the clay crystal which generally
varies from 1.0-1.3 nm. The presence of positive ions on the surface also
makes the clay crystal (002) planes hydrophilic and thus incompatible
with many polymers. The organophilicity of the clay crystal (002) planes
can be increased by exchange of these ions with organic cations (alkyl
ammonium ions) (Alexander and Dubios, 2000). Ion exchange and surfactant
treatment are not absolutely effective in commercially modified clay.
Generally, two clay species result: unmodified clay (Bafna et al.,
2003) with small layer spacing on the order of 1 nm and onium (alkyl ammonium)
modified clay with a layer spacing on the order of 1-5 nm (Brindley and
Brown, 1980) depending on the number of carbon atoms in the chain of the
onium ion. The onium modified clay is thought to retain planar 0.95 nm
thick trilayers of layered silicates. The main evidence for this comes
from the existence of a stacking period after intercalation. Two diffraction
peaks are generally observed from these trilayer structures; a long-period
like layer spacing, (002), oriented normal to the layer face and a weaker
(110)/(020) combined orthogonal reflection at about 0.44 nm. The 0.44
nm reflection should always orient orthogonal to the layer spacing peak
for planar alumino silicate tri-layers (Takahashi et al., 2002).
Benefits of clay over other nano-sized conventional filler: It
is of interest to know the advantages of clay over other nano-sized filler
like nano-fibers. The most important characteristic of filler that possibly
plays an important role in deciding its reinforcing ability is its surface
area to volume ratio (A/V). In order to determine the ratio for these
fillers let us consider nano-fibers (Diameter = D and length = L) and
clay layers (thickness = D, length and width = L) as shown in Fig.
Surface area of each fiber,
A nano-fiber = π D2/2
Volume of each fiber,
V nano-fiber = πD2
Therefore from Eq. 1 and 2,
|| Schematic of (a) nano-fiber and (b) clay layer
A nano-fiber/V nano-fiber = 2/L
~ 4/D (since L>>>>D)
Surface area of each clay layer,
Volume of each clay layer,
Therefore from Eq. 3 and 4,
A clay/V clay = 4/L + 2/D ~ 2/D
Comparing A/V value of a nano fiber (Eq. 3) and a clay
layer (Eq. 6), it is clear that the ratio for nano-fiber
is two times higher than that for clay. This means that for a fixed volume
fraction of filler, the nano-fibers would have higher surface area of
the filler exposed to the polymer matrix and thus would have higher reinforcing
ability than the clay layers. Higher reinforcing ability of the nano-fibers
gives them an advantage over clay layers. On the other hand, when oriented
strongly in a particular direction in the composite, the ability of clay
layers to reinforce the composite bi-axially gives them an advantage over
nano-fiber which just reinforces the composite uni-axially (Fig.
Thus applications where biaxial reinforcement is required would surely
prefer clay as filler. Also properties like barrier, flame retardancy
etc. in which hindrance to the transport of gases or fluids along a particular
composite direction is required, requires large amount of filler surface
area to be normal to that direction of the composite. For highly oriented
filler, this area would be significantly larger in a clay composite than
in a nano-fiber composite. Therefore even thought the nano-fibers have
higher A/V ratio, factors like biaxial reinforcing ability and higher
ability to prevent the transport of gases and fluids, makes clay highly
preferable filler than nano-fibers. For exactly the same reasons, clay
was used as reinforcement in our studies.
Polymer-clay interaction: For the greatest property enhancement
in PLSN systems it is generally believed that the clay layers should disperse
as single platelets throughout the polymer matrix. This is termed exfoliation.
Applying tensile or compressive load to a polymer nanocomposite would
produce different values of stress, in the polymer matrix and in the clay
layers, due to a significant difference in their modulus. Dispersion of
clay as single nm sized layers in the polymer matrix prevents stress concentration.
To attain such dispersion of clay platelets the polymer should first penetrate
between the clay platelets. This intercalation is possible if both the
polymer and the clay layers have polar groups that have favorable interaction.
Depending on the interaction between the clay and the polymer and the
clay loading (Giannelis et al., 1999; Fong et al., 2002)
different regimes of dispersion are expected (Fig. 4).
If the polymer and clay are incompatible, the clay platelets remain as
large stacks (chunks) without any polymer chains entering the region between
the clay platelets (clay gallery) (Fig. 4a). Here the
clay persists as chunks of face to face stacked layers throughout the
polymer matrix. This incomplete and non-uniform dispersion of clay layers
creates large regions of pure polymer in the nanocomposite leading to
poor properties. If the polymer enters the clay gallery but the platelets
still remain as a stack, the system is said to be intercalated (Fig.
4b). Intercalation generally increases the d-spacing of the clay platelets
by around 0.5-1.5 nm which is evident from the shifting of the clay stacking
peak to smaller angles in an x-ray diffraction pattern. Such intercalated
systems have regions of very high and very low reinforcement concentration
limiting stress transfer throughout the composite, as explained earlier,
giving comparatively less than optimal reinforcement (Fornes et al.,
2001; Varlot et al., 2002; Kojimae et al., 1993a). Exfoliated
systems (Fig. 4c) are formed when the polymer enters
between the clay platelets and force them apart so they no longer directly
interact with each other.
Ability of (a) clay layers and (b) nano-fibers to reinforce
a composite biaxially and uni-axially respectively. Arrows indicate
the direction of reinforcement
Evidence of exfoliation comes from the weakening or absence of the clay
stacking peak, (002), in the x-ray diffraction pattern. The highest degree
of property enhancement is obtained only when the clay layers are completely
and uniformly dispersed or exfoliated in the polymer matrix (Fornes et
al., 2001; Varlot et al., 2002).
Different nanocomposite systems obtained depending on
the dispersion of the clay platelets in the polymer matrix. (a) Incompatible
system (chunks of clay platelets), (b) Intercalated system (tactoids
of clay platelets) and (c) Exfoliated system (single clay platelets)
(Brune and Bicerano, 2002) have proposed that exfoliated platelets could
have a high degree of flexibility based on their ability to produce large
bending moments due to high aspect ratio.
Microscopic study made by Schon et al. (2002) evidenced the bending
of clay layers in TEM micrographs. However, the presence of the (110)
reflection in careful studies on single clay layers (Takahashi et al.,
2002) seems to contradict the TEM micrographs of Schon et al. (2002)
since it would be impossible to obtain long-range 3-d ordering in the
plane of the exfoliated platelets if they were bent. Additionally, the
reason for a continuous curvature of the clay platelet proposed by Brune
and Bicerano (2002) rather than a folded or crumpled structure remains
vague. The curvature observed in TEM may reflect the sample preparation
technique where thin slices normal to the clay platelets are used. Such
thin slices might display continuous curvature in response to residual
stress from the sample preparation procedure. No direct evidence of curved
platelets in their native 3-d state exists in the literature. The mechanical
property of isolated platelets of this type is an open question. If isolated,
exfoliated clay platelets truly display compressive flexibility, then
there may be some advantage to intercalated clay where such flexural rigidity
In the case of polar polymers like nylon and EVA (ethylene vinyl acetate
copolymer) the organophilic surfaces of clay have sufficient interactions
with the polar groups of the polymer to easily produce exfoliated nanocomposites.
On the other hand, the case of non-polar polymers is completely different.
In the case of non-polar polymers like polyethylene (PE), polypropylene
(PP) there is very little or no interaction between the polar modified
clay layers and the non-polar polymer. In such situations one of the following
processing routes are used:
||Adding small amount of a polymer, which is compatible
with both clay and the polymer in which it is dispersed. The polar
polymer enters the gallery first, pushes the clay layers apart and
therefore increases the gallery height. This facilitates the non-polar
polymer to enter the gallery. For example, maleaic anhydride grafted
PP was used as a compatiblizer in PP/clay nanocomposite (Svoboda et
al., 2002). Since it is only the polar polymer, which interacts
with the organo-clay, it is generally preferred to make a master-batch
of the polar polymer and clay so as to attain maximum interactions
between the clay and the polar polymer (Mehta et al., 2004).
This master-batch is then let down into the non-polar polymer making
this route a two step process.
||Polar groups may be added to the backbone of the non-polar polymer
thus increasing the interaction between the polymer and the clay.
This route involves mixing the polymer directly with the organo-clay,
making it a one-step process and thus preferable than the method (a)
described earlier. The explanation of the routes for the synthesis
of a polymer-clay nanocomposite is given further.
SYNTHESIS OF POLYMER-CLAY NANOCOMPOSITES
In situ polymerization: Toyota Motor Company (Japan)
filed the first US patent (#4739007) for the development of nylon-clay
nanocomposites by in situ polymerization route (Okada et al.,
1987). This route involves intercalation of monomers between the clay
galleries and then polymerizing it in situ. To facilitate the polymerization
reaction and intercalation, catalysts are supported on the clay surfaces.
The solution containing catalyst supported on the clay is then introduced
into a high-pressure polymerization reactor along with the monomer. Reaction
is carried out at high temperature and pressure to obtain a polymer nanocomposite.
Various polymer-clay nanocomposites, using polymers like polystyrene (Okamoto
et al., 2001), epoxy (Beker et al., 2002), poly (ethylene
terepthalate) (Sekelik et al., 1999), polyethylene (Gopakumar et
al., 2002; Jin et al., 2002), poly (methyl methacrylate) (PMMA)
(Okamoto et al., 2000) etc., have been synthesized by this route.
Although nanocomposites prepared by this route have promisingly showed
improved properties, the batch size achieved by this route in a laboratory
is limited due to very small reactors. From an industrial point of view,
presence of additives in the system lead to complicated reaction conditions
making the production of these materials very complicated in the large
reactors used in an industry (Feng and Nelson, 2002). These are some of
the reasons that have made the bulk production of the nanocomposites by
this route very unlikely in the industry.
Melt blending: polymer-clay nanocomposites could be developed
by melt blending the polymer and an organophilic clay in a twin-screw
extruder. In this route both polymer and clay are either simultaneously
fed, or separately premixed and then fed, to the twin-screw extruder (Giannnelis,
1993). The heat and shear generated by the screw in the barrel of the
extruder facilitates the intercalation/exfoliation of clay in the polymer
matrix. Various polymer-clay nanocomposites, using polymers like nylons
(Fornes et al., 2001), PET (Davis et al., 2002), PET (Wang
et al., 2001), PP (Usuki et al., 1997) etc., have been
synthesized by this route. Being comparatively easier than the in situ
route, the development of the melt blending route brought polymer-clay
nanocomposites closer to commercialization.
Solution synthesis: Aranda and Ruiz (1992) reported PEO/montmorillonite
nanocomposites can be prepared by dissolving PEO in a suitable solvent
which also swells the montmorillonite. In this method for synthesizing
polymer-clay nanocomposites, the polymer is first dissolved in a solvent
and then modified clay is added to it. The solvent used in this technique
should dissolve the polymer and also swell the clay layers. The mechanism
for the formation of nanocomposites by this technique involves two steps.
1) Swelling of the clay layers by the solvent and then 2) intercalation
of the polymer chains into the expanded clay galleries by displacing the
solvent molecules out of the gallery.
After the solvent is completely displaced out of the galleries, the system
is heated till all the solvent evaporates. On removal of all the solvent
it has been observed that the intercalated clay remains intact, resulting
in polymer-clay nanocomposite. Various polymer-clay nanocomposites, using
polymers like poly-vinyl acetate (Strawhecker and Manias, 2000), PE (Jeon
et al., 1998) and PEO (Choi et al., 2001), have been synthesized
using this route.
Although no studies involving measurements of the nanocomposite properties
after a certain period of time (in order to study the aging behavior of
the nanocomposite) is reported, it is believed that once the clay is dispersed
by a certain distance from the neighboring platelet so that no VanDer
Waals interactions are present, the clay layers would not collapse back
and deteriorate the properties.
Although this route can be used to synthesize nanocomposites from polymers
with little or now polarity, from a commercial point of view, this route
involves use of organic solvents in a large amount, which is environmentally
unfriendly and economically prohibitive. Also it is believed that a small
amount of solvent remains in the final product at the polymer-clay interface
thus creating weaker interfacial interaction between the polymer and the
clay surfaces (Jin et al., 2002).
Dispersion in polymer-clay nanocomposites: Most of the studies
on nanocomposites have used x-ray diffraction and transmission electron
microscopy to study the type of dispersion present in the sample. The
explanation of these techniques with their advantages and disadvantages
are discussed as follow:
Characterization techniques for determining dispersion of clay:
TEM and X-Ray Diffraction (XRD) are the most widely used techniques to
determine the dispersion of the clay platelets in the polymer matrix.
Some studies have used either Small Angle X-ray Scattering (SAXS) or wide-angle
x-ray scattering or both together to get data on dispersion and orientation
of the clay platelets in the polymer matrix.
Transmission Electron Microscopy (TEM): The micrograph is of a
commercially available nylon-clay nanocomposite and was obtained from
the nanocomposite database at Equistar Chemicals (Fig. 5).
The dark lines in the micrograph are the dispersed clay layers in the
polymer matrix. It is seen that the clay is dispersed as a stack of clay
layers with the total stack thickness of approximately 2-5 nm and containing
2-5 clay platelets. Although TEM is a technique that gives a clear picture
of the morphology of the nanocomposite, good sample preparation techniques
for TEM are pivotal. Very thin cross-sections (40-50 nm) of the nanocomposite
are required to get a good quality image. For this reason the nanocomposite
needs to be microtomed cryogenically. Microtoming of a material in to
very thin sections (40-50 nm) and heating of the sections in the TEM column
by the high-energy electron beam could have an effect on the morphology
of the material. Also, TEM gives only a planar projection of the orientation
of the clay platelets (Fig. 5) and thus performing TEM
along a single cross section of the sample could give misleading results
regarding the orientation of the clay platelets in the polymer matrix.
Performing TEM on all the 3 different faces of a sample, even though extremely
tedious, could give a rough idea of the 3-d orientation of clay layers
but still could be misleading as it is very difficult to identify the
sample direction, without making some assumptions, when viewing the cross-section
in the microscope at 100,000 times magnification, at which the clay layers
are clearly visible.
Transmission electron micrograph of a nylon-clay nanocomposite
showing planar projection of the orientation of clay platelets (dark
lines) in the polymer matrix. Micrograph obtained from the nanocomposite
database at Equistar Chemicals.
X-Ray Diffraction (XRD): XRD gives quantitative data on the dispersion
of the clay platelets. The clay platelets are arranged periodically in
an intercalated system and thus a reflection from the clay platelets is
observed in the XRD pattern. As more and more polymer chains enter the
clay gallery, the clay spacing increases, shifting the clay peak to lower
angles (2θ <2°).
The separation of the clay platelets also decreases the periodicity and
hence reduces the intensity of the clay peak. For an exfoliated system
the clay platelets are randomly dispersed and no clay peak is observed
in the XRD pattern. But the lack of a Bragg`s peak in the diffraction
pattern does not necessarily mean that the clay is exfoliated. A disordered
and immiscible sample, or other factors like low concentration of the
clay in the region where the x-ray beam hits a non-uniformly dispersed
sample, could fail to produce a Bragg`s reflection (Morgan and Gilman,
2003). It is also difficult to get diffraction data from thin nanocomposite
films when a reflection mode diffractometer is used. Thus XRD can be used
to characterize as incompatible or intercalated system but could possibly
give misleading data on the exfoliation.
Small and wide angle x-ray scattering (SAXS and WAXS): SAXS and
WAXS when used together give data on both dispersion and orientation of
clay platelets and various other structural features in a polymer-clay
nanocomposite. Scattering data from both, molded samples and thin films
can be obtained. Recent studies have obtained data on the 2-d orientation
of clay platelets and polymer structures in a nanocomposite system, which
could be misleading (Varlot et al., 2002).
Most of the studies present by Hambir et al. (2001), Fornes et
al. (2001), Oya et al. (2000) Beyer and Eupen (2001) and Gilman
(1999) propose that the dispersion of the clay platelets play an important
part in tuning the property enhancements in PLSN systems. For this reason
a review on the factors affecting the dispersion and the mechanism of
property enhancement based on dispersion of clay is presented here.
Factors effecting dispersion of clay platelets: The presence of
positive ions on the surface makes the clay platelet hydrophilic and thus
incompatible with many polymers. The organophilicity of the clay platelets
is increased by exchange of these ions with organic cations (alkyl ammonium
ions) on treating it with ammonium salts (surfactants). These treated
clays are called organically modified layered silicates (OLS`s). The dispersion
of these OLS`s in the polymer matrix depends on a number of factors some
of which are explained here.
Molecular weight of the polymer: Fornes et al. (2001) proposed
a model for explaining the mechanism of exfoliation of clay layers, from
an initially present stack of clay layers, by the stepwise skewing of
the clay platelets, followed by peeling, one-by-one, of the silicate layers
off the silicate stack under the influence of shear forces. A schematic
of the skewing/peeling and diffusion mechanism as proposed by Fornes et
al. (2001) (Fig. 6). The intercalation/exfoliation
of clay platelets was shown to be a combination of two mechanisms the
breaking up of large clay stacks into smaller stacks of fewer and fewer
clay platelets under the influence of large shear forces (Fig.
6a) and the diffusion of the polymer molecules into the clay galleries
(Fig. 6b). They also showed that higher the molecular
weight of the polymer matrix, higher was its melt viscosity; greater was
the stress exerted on the stack of clay platelets and lesser was the number
of platelets left in the stack.
||Mechanism of intercalation/exfoliation of the clay platelets
by the polymer (Fornes et al., 2001)
Although the mechanism of intercalation/exfoliation by breaking of large
clay crystals into smaller ones (Fig. 6a) and then peeling
of each platelet one after one seems possible, the bending of the ends
of clay layers (Fig. 6b) in order to allow the intercalation
of polymer chains seems unlikely. Most likely the whole clay platelets
at the top and at the bottom of a clay crystal would just separate out
one after another by skewing under the influence of shear force since
this would require less energy than that required for bending the ends
of the clay layers and hence their ionic bonds with the neighboring platelets.
Increase in the molecular weight decreases the diffusion of polymer chains
(D ~ 1/M2) (Strobl, 1997) into the clay galleries and hence
should decrease the rate of hybrid formation. Thus, in a nanocomposite
system, the rate of hybrid formation depends on the type of mechanism,
which dominates. For example, Ishida et al. (2000) observed a decrease
in the degree of intercalation/exfoliation on increase in the molecular
weight of the base resin.
They prepared the nanocomposites by manually mixing the polymer melt
and clay. Due to the lower degree of shear involved, the mechanism of
the diffusion of polymer chains dominated the intercalation process and
hence the degree of intercalation/exfoliation decreased with the increase
in molecular weight. Fornes et al. (2001) observed an increase
in the degree of intercalation/exfoliation with molecular weight in nylon-clay
nanocomposites. The nanocomposites were made by extruding a mixture of
polymer and clay through a twin-screw extruder. Due to the higher degree
of shear involved the breakup of large stacks into smaller ones took place
and hence increased the rate of hybrid formation. Thus the molecular weight
of the base resin affects the rate of hybrid formation by two different
mechanisms and the actual rate depends on the combinatory effect of both
Thermodynamic interactions: Vaia and Giannelis (1997a) proposed
a mean-field model to explain the intercalation of polymer molecules in
an organo-clay. They propose that changes taking place in both entropy
and enthalpy by the addition of organo-clay into the polymer matrix play
an important role in determining the degree of intercalation/exfoliation
in the nanocomposite system.
From an entropic point of view Vaia and Giannelis, (1997a) propose that
the intercalation of polymer chains in the clay gallery decreases its
entropy due to polymer confinement and thus decreases the overall entropy
and thus the free energy, of the system which is unfavorable for further
polymer intercalation. But Vaia and Giannelis, (1997a) also propose that
this decrease in entropy can be balanced by the increase in entropy of
the alkyl-ammonium chains on the surface of the clay layers due to increased
gallery height. The increase in the entropy depends on the gallery height
which in turn depends on the interlayer packing density of the alkyl ammonium
(aliphatic) chains (Vaia and Giannelis, 1997b). Interlayer packing density
is the number of aliphatic chains of the surfactant present per unit area
of the surface of the silicate layer. At very low packing densities the
aliphatic chains display mono/bilayer arrangement while at very high packing
densities they display solid-like fully extended arrangement (Vaia and
Giannelis, 1997b; LeBaron et al., 1999).
At both low and very high packing density the aliphatic chains have comparatively
less conformational freedom and thus the total entropy increase is not
high enough to balance the decrease in entropy by polymer confinement
and is thus unfavorable for polymer intercalation (Vaia and Giannelis,
1997a). At intermediate range of packing densities the chains adopt pseudo-trilayer
arrangements which have comparatively higher conformational freedom and
thus balance the decrease in entropy by polymer confinement, resulting
in a net entropy change near zero (Vaia and Giannelis, 1997a).
Thus the hybrid formation by melt intercalation now depends on the enthalpic
interactions between the polymer and clay surfaces which can be increased
by polymer and clay surface functionalization (Vaia and Giannelis, 1997b).
By experiments they also showed that the outcome of polymer intercalation
depends on the silicate functionalization and constituent polar interactions
between the clay and the polymer.
Polymer-clay interaction: As explained in the earlier section
it was shown by (Vaia and Giannels, 1997a) that functionalization of the
clay surfaces is very critical for polymer intercalation into the clay
galleries. Presence of functional groups on the surface of the clay layers
easily intercalate polar polymers but do not favor intercalation of non-polar
polymers like polyethylene (PE) and polypropylene (PP). Addition of a
compatibilizer like maleatedpolyolefin oligomer increases the interaction
between the clay surface and the non-polar polymer. This increase in interaction
originates from the strong hydrogen bonding between the maleic anhydride
groups and the oxygen atoms on the clay surfaces (Liu and Wu, 2001). Increased
interaction between the two, acts as a driving force for the intercalation
of the compatibilizer into the clay galleries. The bulky nature of the
maleated-polyolefin oligomer increases the clay-spacing and decreases
the interactions between the clay layers which in turn favor the intercalation
of the base resin (PE/PP). Increasing the polarity of the compatibilizer
increases the interactions between the clay and the polymer and hence
the degree of intercalation/exfoliation (Vaia and Giannelis, 1997b).
Property enhancements based on clay dispersion
Mechanical properties: Varlot et al. (2002) studied
the injection molded samples of both intercalated and exfoliated nylon
6-clay nanocomposites. Both the systems showed an increase in the storage
modulus, which increased monotonically with the clay content. Compression
modulus and strain was found to be different along different faces of
the sample and was maximum for the faces along the injection axis of the
sample. The compression modulus and strain along the same face increased
with increase in the filler content. An exfoliated system had higher values
of tensile modulus than an intercalated one and the modulus increased
with increase in the filler content. The variation in the compression
modulus and strain along different directions could be due to the preferential
orientation of the clay platelets along the injection axis of the sample.
The higher values of the tensile modulus in the exfoliated sample as compared
to the intercalated sample could be due to the increased surface area
of the clay platelets in contact with the polymer.
Brune and Bicarano (2002) developed a model to predict the effects of
incomplete exfoliation on the tensile modulus of nanocomposite systems.
In an incompletely exfoliated nanocomposite the stacks of clay layers
(tactoids), with polymer intercalated in their galleries, are called pseudoparticles
in their study. For nanocomposites containing pseudoparticles, they modified
the Halpin-Tsai equation as the stack of clay layers now behave as a single
large particle which has decreased aspect ratio and decreased effective
elastic modulus. Due to the decreased aspect ratio and elastic modulus
of the filler, the relative modulus of the nanocomposite having pseudoparticles,
with respect to a nanocomposite with exfoliated clay layers is lower.
This model thus theoretically supports the experimental observation of
Varlot et al. (2002) that exfoliated nanocomposites have significantly
higher properties than a intercalated nanocomposite.
Kojima et al. (1993b) studied the mechanical properties of nylon
6-clay nanocomposites prepared using organically modified montmorillonite
(NCH) and synthetic (NCHP) clays. TEM showed that both nanocomposites
were exfoliated. The nylon 6-clay hybrid (NCH) was superior in strength
and modulus to nylon 6. The flexural strength of NCH at 120 °C was
double than that of nylon 6. The flexural and tensile modulus was 4 times
and 3 times that of nylon 6 respectively. This contradicts (Brune and
Bicerano, 2002) statement that the clay platelets are flexible. Nylon
6-clay hybrids (NCHP) prepared using synthetic clay had properties better
than nylon 6 but not better than NCH. This difference in the mechanical
properties could be due to the smaller aspect ratio of the synthetic clay
as compared to montmorillonite clay and the direct relationship between
the aspect ratio and modulus as shown theoretically by (Brune and Bicerano,
Degradation properties: Zanetti et al. (2001) investigated
the degradation properties of PP-clay nanocomposites. A 50 °C increase
in the temperature at which thermal degradation starts was observed as
compared to the homopolymer. A decrease in the rate at which degradation
and weight loss proceeded was also observed. They relate this increase
in the thermal stability in the nanocomposites to the decrease in rate
of transport of volatile products in and out of the sample. The decrease
in the transport could be due to the labyrinth effect (complex arrangement
of the silicate layers which increases the tortuous path) that increased
the barrier to the flow of the degradation volatiles and physical adsorption
of the volatile products on the surface of the silicate layers.
Liang and Yin (2003) also studied the degradation properties of poly
(etherimide) (PEI)-clay nanocomposites. From XRD and TEM results they
claim that the montmorillonite was exfoliated in the polymer matrix. They
observed a moderate increase in the temperature of onset of thermal degradation
by addition of organo-clay to the nanocomposite. The onset temperature
of thermal degradation increased from 522 °C for PEI to 534 °C
for the PEI-clay nanocomposite containing about 10 wt. % organo-clay.
They mention that montmorillonite has higher thermal stability and the
layer structure of montmorillonite exhibited a greater barrier to the
flow of small molecules generated during the thermal decomposition of
Thermal properties: Liang and Yin (2003) also studied the thermal
stability of PEI and PEI-clay nanocomposites. They observed a significant
decrease in the coefficient of thermal expansion (CTE) of PEI by addition
of clay to it. The CTE of PEI decreased from 2.37x10-5 K-1
for PEI to 1.33x10-5 K-1 for PEI containing 10 wt%
organo-clay. They propose the reduced segmental motion of the PEI matrix
by addition of clay to it as the reason for the decrease in the CTE.
Yoon et al. (2002) studied the thermal expansion behavior of nylon
6 nanocomposites. TEM analysis showed that the clay layers were exfoliated
in the polymer matrix. They studied the coefficient of thermal expansion
for injection molded Izod bars. They observed a decrease in the CTE along
the Flow Direction (FD) and the Transverse Direction (TD) when compared
to nylon 6 containing no clay. They observed that the CTE decreases more
along the FD than along the TD and suggest non-uniform platelet orientation
along the FD, since perfect alignment of the clay layers in an isotropic
polymer matrix should yield equal CTE along both FD and TD. They also
showed that nanocomposites made from high molecular weight polymer had
higher degree of exfoliation (higher aspect ratio) and thus lower CTE
and the nanocomposites made from lower molecular weight polymer had lower
degree of exfoliation (lower aspect ratio) and thus higher CTE even though
it was lower than the base resin.
The effect of molecular weight on the degree of exfoliation has been
Flame resistance: Beyer and Eupen (2001) and Gilman (1999) investigated
the flammability of ethylene-vinyl acetate (EVA)-clay nanocomposites.
The EVA-clay nanocomposites were found to have better flame retardant
properties. It was observed that the intensity of the peak of heat release
decreased by 47% for a nanocomposite containing 5 wt. % of the modified
montmorillonite when compared to the pure EVA. The decrease in the intensity
of the peak of heat release indicates a reduction of the burnable volatiles,
which indicates the flame-retardant effect due to the presence of clay.
Such properties are further improved by the fact that the peak of heat
release is spread over a much longer period of time. This improvement
in the flame-retardant properties is due to char formation during the
nanocomposite combustion. The clay layers act as reinforcement to the
char produced and thus forms a thick insulating, non-burning material
that reduces the emission of the volatile material into the flame area.
The silicate layers thus play an active part in decreasing the transport
of volatile products and hence improving the flame-retardant properties.
Barrier properties: The enhancement of barrier properties is believed
to depend on the degree of exfoliation of the clay platelets. In the fully
exfoliated state, individual clay platelets have the highest aspect ratio
possible and thus the highest barrier improvement is expected. Kojima
et al. (1993a) studied the barrier of nylon-montmorillonite and
nylon-saponite (synthetic clay) nanocomposites to water. Both nylon-montmorillonite
and nylon-saponite nanocomposites had better barrier to water than nylon
homopolymer. The higher aspect ratio of montmorillonite compared to saponite,
made nylon-montmorillonite nanocomposites more resistant to water absorption
than nylon-saponite nanocomposites.
Most of the studies explained above have related the property enhancement
to the dispersion and/or the aspect ratio of the clay platelets. Properties
mentioned above depend on the transport of some material in and out of
the sample and thus, the enhancement in these properties will depend on
the orientation of the clay platelets in the sample. For this reason recent
studies have proposed that along with the dispersion, the orientation
of the clay platelets should be considered to understand the true relation
between morphology and property enhancement.
Rheological properties: Understanding the rheological properties
of PLSN`s is of significant interest for the design of customized polymer-clay
systems with enhanced mechanical, thermal, flame resistance and barrier
properties. Various factors like polymer-filler interaction, filler-filler
interaction and the dispersion and orientation of filler could influence
the viscoelastic properties of the polymer matrix (Krishnamoorti et
al., 2001). The response of the presence of clay in the polymer matrix,
either as intercalated or exfoliated structures, to external flow is essential
in their processing (Krishnamoorti and Yurekli, 2001). For this reason
recent studies have focused on studying the rheological properties (Krishnamoorti
et al., 2001; Galgali et al., 2001; Lele et al.,
2002) of the polymer matrix on addition of clay to it. Krishnamoorti et
al. (2001) studied the response of polystyrene-1,4 polyisoprene block
copolymer/clay nanocomposites to steady shear as a part of their study
about the effect of clay on the rheological properties of polymer-clay
nanocomposites. The steady shear data for the intercalated nanocomposites
showed non-Newtonian behavior in the low shear rate region where the polymer
without clay shows Newtonian behavior.
Increasing the clay content was shown to enhance the shear-thinning behavior
of the nanocomposite system. Significant improvement of the zero shear
viscosity in the nanocomposite system was observed. Additionally, they
observed that the viscosity of the nanocomposite systems approached that
of the polymer at high shear rates. Significant improvement of the zero
shear viscosity in the nanocomposite system was attributed to the confinement
of the polymer chains in the clay galleries. The enhanced shear thinning
behavior and the viscosities comparable to that of the polymer at high
shear rates was attributed to the ability of the anisotropic silicate
layers to orient with the application of shear flow. This ability of the
anisotropic clay layers to orient with the shear flow was observed by
Krishnamoorti and Giannelis (1997). Similar effects of the presence of
clay on the rheological properties were observed by Galgali et al.
(2001) and Lele et al. (2002) on different polymer systems.
ORIENTATION STUDIES OF POLYMER-CLAY NANOCOMPOSITES
Although studies Hambir et al. (2001) and Zhang et al.
(2000) have mentioned that true nanocomposites and significant property
enhancements are obtained only when the clay disperses as single platelets
(exfoliation) in the polymer matrix, there are some studies (Nam et
al., 2001) which have observed enhancements in properties just by
intercalating the clay platelets with the polymer chains. Thus the type
of dispersion of the clay platelets into the polymer matrix does not seem
to be the only governing factor in tuning the property enhancements in
nanocomposites. Recent studies mention that along with dispersion, orientation
of the clay platelets plays a major role in tuning some property enhancements
in PLSN systems. Kojima et al. (1995) concluded that the orientation
of the clay platelets affected the strength of the nanocomposite along
different sample directions. Krishnamoorti et al. (2001) proposed
that the orientation of the clay platelets could affect the viscoelastic
properties of the nanocomposite. Recently a few studies have focused on
the effect of shear on the orientation of the clay platelets and the polymer
unit cells in PLSN systems.
Kojima et al. (1995) studied the effect of shear on the orientation
of clay platelets and polymer unit cells as a function of depth in a 3
mm thick injection molded samples of nylon-clay nanocomposite. Depending
on the shear involved the clay platelets and the unit cell (020) or (110)
lattice planes oriented in different directions. Due to the high shear
involved in the region of the sample close to the surface of the mold,
both clay platelets and polymer unit cell (020) or (110) lattice planes
were found to orient along the flow direction. In the bulk of the sample
the clay platelets and the polymer unit cell (020) or (110) lattice planes
were found to orient perpendicular to each other due to lower shear rate.
The clay platelets were found to govern the orientation of the polymer
unit cells due to hydrogen bonding between the ammonium cations at the
end of nylon-6 molecules and the ionic sites on the montmorillonite monolayer.
Fong et al. (2002) studied clay dispersion and orientation in
nylon-clay nanocomposite films and fibers using XRD and TEM. Films were
cast and fibers were electrospun from solution. The effect of shear on
the orientation of clay platelets and polymer unit cells in electrospun
nylon-clay nanocomposite fibers was also studied. They found that the
fibers had layered silicates aligned with layer normal perpendicular to
the fiber axis and polymer crystallite (020) planes aligned with normal
parallel to the fiber axis implying orthogonal orientation between clay
platelets and polymer crystallite (020) planes.
RHEOLOGY AND ORIENTATION OF COLLOIDAL DISPERSION OF CLAY PLATELETS
Understanding the rheological properties of PLSN`s is important in order
to control orientation of platelets and their structure-property relationship.
For this reason recent studies have focused on studying their rheological
properties. Addition of clay and its interaction with the polymer was
shown to have a significant effect on the rheological properties. The
nanocomposites showed higher melt viscosities and greater shear thinning
(Galgali et al., 2001; Lele et al., 2002). Thus the literature
requires a true understanding of the role played by the dispersed phase
(clay layers) on the polymer melt. A explanation of the effect of the
addition of spherical or non-spherical colloidal particles on the rheological
properties of dispersion as follows:
Einstein proposed an additive expression for the viscosity of a dilute
dispersion of solid spheres based on Stokes law (Strobl, 1997). According
to his model,
η´ = η (1 + 2.5 φ
||Volume fraction of the dispersed phase
|η` and η
||Viscosities of the suspension and the solvent respectively.
Thus an increase in the viscosity of the dispersion on addition of the
solid spheres is expected. Changing the shape of the dispersed particles
from spherical to ellipsoidal or similar shapes in which one of the axes
is longer than the other (e.g., clay platelets), makes the model more
complex. For a suspension of asymmetric particles the viscosity would
depend on the orientation of the particles with respect to the flow direction
(Simha, 1940). Due to this dependence of the viscosity of the suspension
on the orientation of the particles modified the Einstein`s equation for
a suspension containing asymmetric particles. The equation proposed by
Simha (1940) and Larson (1999) for the macroscopic viscosity, η`,
of the suspension of as asymmetric particles is written as follows.
where, v according to Eq. 7 for solid spheres is 2.5
and is larger for asymmetric particles. For disc-shaped particles v is
written as follows (Simha, 1940; Larson, 1999).
A = The aspect ratio, i.e., ratio of the diameter (D) to the thickness
(t), of the particle.
Inclusion of v (v> 2.5) along with the volume fraction φ in the
Einstein`s equation could mean that the presence of asymmetric particles
in a suspension possibly increases the effect of the volume fraction on
the viscosity of the suspension as compared to that by the presence of
spherical particles of same volume fraction. This behavior of asymmetric
particles can be explained as follows.
v is directly proportional to the aspect ratio as seen in Eq.
9 and so it can be written as v ~ A = D/t. The volume fraction of
the filler is φ = D2 tN/φs where, N is
the number of asymmetric particles and φs is the total
volume of the suspension. Product of φ and v yields V*~ D3N/φs
which is the volume fraction of spheres of diameter D in a suspension.
The product of v and φ therefore increases the effect of volume fraction
on the viscosity of the suspension since the asymmetric particles of diameter
D now possibly behaves as having occupied the volume equal to that which
would have been occupied by spheres of the same diameter (Fig.
The morphology of a polymer-clay nanocomposite showing
the increase in the occupied volume fraction of the filler due to
its asymmetric nature
Simha`s modification to Einstein`s equation takes care of the asymmetric
nature of ellipsoidal filler particles. In a shearing flow if the particle
long axis is inclined to the plane of deformation (the plane parallel
to the flow) then the particle will rapidly rotate initially and then
slowly till its long axis is parallel to the plane of deformation. The
angle of rotation depends on the quantities like shear rate, the time
of shearing and the aspect ratio of the particles (Tanford, 1967). The
Jefferies orbit that relates these quantities for ellipsoidal filler particles
Tan θt = p tan [shear rate*
t/(p + 1/p)] +tanθ0
where, θt, θ0 are the angles of the axis of symmetry
of the filler particles measured in the clockwise direction from the flow
direction at time t and time zero respectively and p is the aspect ratio
(Tanford, 1967). Therefore control of the shear rate and the time of shear
can control the amount of rotation/orientation.
The Jefferies equation is good only for dilute concentrations of the
dispersed particles. At higher loading of clay platelets in a PLSN system,
the tendency of the particles to form a percolating network arises (Galgali
et al., 2001; Lele et al., 2002). Galgali et al.
(2001) studied the creep behavior of PP-clay nanocomposite containing
9 wt% clay as apart of their study on the rheological response of polymer
nanocomposites. They propose that the frictional interaction between the
clay layers could be one reason for the solid like rheological behavior
of the nanocomposite systems. Although no evidence either by microscopy
or XRD was shown, they propose that at high clay loadings (~9 wt. %) small
amount of clay layers exfoliate and form networks between the intercalated
clay tactoids finally producing a percolating network. By studying the
response of viscosity of the nanocomposite system to shear stress they
observed that, the nanocomposite creeps at low stresses in a Newtonian
manner, possibly due to the presence of percolating network, followed
by apparent yielding at about 1000 Pa, in which the viscosity drops by
3 orders of magnitude in a very narrow range of shear stress. They also
propose that above the yield stress, the percolating structure starts
to break down and the clay layers start to align along the direction of
stress which causes the material to flow to greater extents. Thus to study
the orientation in PLSN`s the stress on the material should be greater
than the apparent yield stress.
Opposing this orientation, randomization of these particles can take
place by Brownian motion under the influence of temperature. The ratio
of shear energy to thermal energy is a measure of the tendency to thermally
randomize. This ratio is called the Rotational Peclet number (Pe) which
is given as:
where, τ is the shear stress, d is the diameter of the disc shaped
particle, kB is the Boltzmann`s constant and T is the temperature.
Particles will orient with shear only when Pe>1 (Tanford, 1967). Therefore,
for clay platelets of average diameter 500 nm and considering a temperature
of 200 °C, the platelets will randomize due to Brownian motion if
τ<10-5 Pa. In a twin-screw extruder, which is the most
commonly used instrument to process polymers, the stress acting on the
material is high (>>105 Pa) and hence the particles will
orient and the degree of orientation can be controlled by controlling
the shear rate. Processing parameters which affect the shear rate are
screw speed, shearing time, temperature of the melt, die-gap and gap between
the screw and the barrel etc. Thus an extruder can be used to study the
effect of the effect of the processing conditions on the orientation and
the property enhancements. Although studies of the rheology of PLSN`s
are present in the literature, the literature lacks studies relating the
effect of the processing conditions on the orientation and the property
The nanocomposites have already been used widely in the various fields
of injection molding, e.g., engine cover, timing belt cover, oil reservoir
tank and fuel hose in automobile industries, floor adjuster and handrail
in the construction fields and various connectors in the electrical fields.
Nanocompsite nylon6-Clay Hybrid (NCH) shows a high modulus and high distortion
temperature (Okada et al., 1987). The timing belt covers made from
NCH by injection molding was the first example of industrialized use of
polymer-clay nanocomposites. NCH has also a high gas barrier property
because of the nanometer level dispersion of silicate layers (Messersmith
and Giannalis, 1995), so it has a wide range of applications in the food
packaging films. As mentioned above, the increased mechanical properties
and dimension stability makes the Nanocomposites convenient to be used
as high value construction materials.
They are highly stable against aggressive chemicals, so they can also
be implemented in corrosive protective coatings.
Due to the decreased permeability for gases and water, as well as for
hydrocarbons, they have a wide range of applications in packaging and
automotive industries. In high temperature areas, such as internal combustion
engines, because of good thermal stability, flame retardancy and HDT;
nanocomposites are more attractive and promising than other conventional
These materials have a good perspective of application for the near future
in daily life. Through the nano-clay reinforcement we expect a new dimension
in the polymer technology. The production of the high-tech composites
like carbon-carbon composites is extremely expensive and labour-intensive,
therefore the art of composites may be considered as another alternative
solution. By the extrusion technology, lower labour-intensive mass production
lines are expected.
Poor barrier to hydrocarbons and lower stiffness to impact ratio of polyethylene`s
has restricted its application in certain packaging/handling and automotive
applications respectively. Reports on improvement in mechanical, thermal,
barrier and flame retardant properties of a polymer by addition of 2-6
wt. % organo-clay have drawn tremendous attention due to significant property
enhancement without a significant increase in weight. This modified thermoplastic
system is called polymer layered-silicate nanocomposite system.
Depending on the interaction between the polymer and the surface of the
organoclay and few other factors like the polymer molecular weight, type
of treatment on the surface of clay layers and the processing conditions,
different degrees of dispersion and thus different levels of property
enhancement are obtained. The dispersion of the clay layers can be quantitatively
and qualitatively studied using XRD and TEM respectively with each of
the techniques having their own merits and demerits.
Most of the previous studies have related the property enhancement to
the dispersion of the clay platelets. Properties like barrier, thermal
stability and flammability depend on the transport of some material in
and out of the sample and thus, the enhancement in these properties will
depend on the orientation along with the dispersion of the clay platelets
in the sample. For this reason recent studies have started focusing on
the effects of both dispersion and orientation on the property enhancement.
In injection molded nylon-clay nanocomposite samples, Kojima et al.
(1995) found that the polymer crystallites either align parallel (high
shear region) or perpendicular (low shear region) to the clay platelets.
Contrary to Kojima et al. (1995) observation, Fong et al.
(2002) observed that polymer crystallites align perpendicular to the clay
platelets in nylon-clay electro spun fibers where exceedingly high shear
rates are expected. Varlot et al. (2002) observed that in intercalated
nylon-clay nanocomposites, the clay platelets aligned with normal both
parallel and perpendicular to the thickness of an injection molded sample
consistent with Kojima et al. (1995). Although it is clear that
the polymer lamellae align in different directions depending on the type
of deformational cumulative shear strain and shear rate, the relationship
of clay platelet orientation to the orientation of other structural units,
such as the polymer unit cells and polymer lamellae still remains unclear.
The application of nanocomposites is currently limited to polar polymer
like nylons, EVA`s (ethylene-vinyl acetate) etc. In order for a major
impact PLSN`s should be applied to non-polar systems like polyolefins
which are the most widely consumed thermoplastics. As mentioned earlier,
polyolefins being non-polar show poor compatibility with modified clays.
Many researchers (Wang et al., 2001; Kodgire, 2001) reported on
the dispersion of clay platelets in polyolefins by addition of a compatibilizer
such as maleated polypropylene or polyethylene. Although previous
studies showed the effect of compatibilizer on property enhancement in
polyolefin nanocomposites, the literature lacks a clear picture of the
effect of compatibilizer on the orientation/dispersion of the clay platelets
and the effect of this clay orientation on the orientation of other structural
units like, polymer unit cells and polymer lamellae.
Although studies on the rheology of PLSN`s are present in the literature
it lacks studies relating the effect of the rheology and the processing
conditions on the orientation and the property enhancement.
Polymer/clay nanocomposites are materials that display rather unique
properties, even at low clay content, by comparison with more conventional
mineral-filled polymers. Nanocomposites have a number of advantages over
traditional polymer composites. Conventional composites usually require
a high content (>l0 wt%) of the inorganic filler to impart the
desired mechanical properties. Such high filler levels increase the density
of the product and can cause deterioration in properties through interfacial
incompatibility between the filler and the organic matrix.
Processability also worsens as filler content increases. In contrast, nanocomposites
show enhanced thermal and mechanical properties with even a small amount of
added clay because the nanoscale dimensions of the clay particles yield a large
contact area between the polymer and the filler. The structure of clays, with
layers of high aspect ratio, also imparts excellent barrier properties, which
in turn provides low gas permeability and enhanced chemical resistance and flame
retardancy. This new type of materials, based on smectite clays usually rendered
hydrophobic through ionic exchange of the sodium interlayer cation with an onium
cation, may be prepared via various synthetic routes comprising exfoliation
adsorption, in situ intercalative polymerization and melt intercalation.
The whole range of polymer matrices is covered, i.e., thermoplastics, thermosets
Nanocomposites are subject of current interest because of their unusual
magnetic, optical, electronic properties, which often different from their
bulk properties. The reasons for these are confinement of electronic and
vibrational excitation, quantum size effect and large surface to volume
ratio. Although Nanocomposites have received attention from both theoretical
and experimental standpoints, the greatest challenge at present is to
find out an effective synthesis procedure. The fundamental challenges
in nanostructured materials are ability to control the scale (size) of
the system, understand the influence of the size of building blocks in
nanostructured materials as well as the influence of microstructure on
the physical, chemical and mechanical properties of this material and
transfer of developed technologies into industrial applications including
the development of the industrial scale of synthesize methods of nanomaterials
and nanostructured systems.